In an AC motor which employs a three-phase alternating current drive source, in some cases, the drive of such an AC motor is controlled by supplying voltage of non-sinusoidal waveform such as stepped waveform to the motor. Such a voltage of non-sinusoidal waveform contains a higher harmonic component. A loss resulting from the higher harmonic component in the supply voltage ultimately produces heat, which causes heat operation in the motor.
In general, the AC motor has both high-speed output characteristics to generate high output at high-speed revolution and low-speed output characteristics to generate high output at low-speed revolution. The high-speed output characteristics or the low-speed output characteristics are attained by varying the number of turns of primary winding in the motor or by selecting between Y-connection and .DELTA.-connection.
Heat generated due to the higher harmonic component contained in drive voltage of the motor presents a problem particularly when the motor is operated with the connection for high-speed output characteristics. Thus, in order to suppress the generation of heat due to the higher harmonic components as described above, a reactor is interposed between the power source and the motor to attenuate the higher harmonic component contained in the drive voltage. The reactor is effective in suppressing the generation of heat in a rotor and a stator of the motor at high-speed revolution, whereas impedance of the reactor causes a decrease in the amount of current supplied to the primary winding at low-speed revolution, resulting in reduction of output.
With reference to FIGS. 8A and 9B, description will first be given of a method for attaining two kinds of characteristics, that is, characteristics for high-speed operation and those for low-speed operation, by switching over primary winding connection in an AC motor, which employs a three-phase alternating drive source.
Referring to FIG. 8A, primary winding comprises U-phase winding 1, V-phase winding 2 and W-phase winding 3 of a motor, which are set up in the form of a Y-connection, and this Y-connection is adapted to attain the characteristics for low-speed operation. The U-phase winding 1 has U and X terminals, the V-phase winding 2 has V and Y terminals, and the W-phase winding 3 has W and Z terminals. The X, Y and Z terminals of the windings are interconnected. Then, U, V and W terminals on a power source side 31 are respectively connected to the U, V and W terminals of the windings 1, 2 and 3 set up in the form of the Y-connection.
FIG. 8B shows .DELTA.-connection switched over from the connection (Y-connection) of the windings shown in FIG. 8A. This .DELTA.-connection is adapted to attain the characteristics for high-speed operation. In FIG. 8B, the U, V and W terminals are respectively connected to the Z, X and Y terminals. Then, the U, V and W terminals on the power source side 31 are respectively connected to the U, V and W terminals of the windings 1, 2 and 3 in .DELTA.-connection.
Next, referring to FIGS. 9A and 9B, a description will now be given of a method for attaining two kinds of characteristics, that is, characteristics for high-speed operation and those for low-speed operation, by switching over a voltage application terminal of a primary winding in an AC motor, which employs a three-phase alternating drive source.
Referring to FIG. 9A, first and second U-phase windings 10, 11 connected in series, first and second V-phase windings 12, 13 connected in series and first and second W-phase windings 14, 15 connected in series are in Y-connection. Then, a terminal U1 of the first U-phase winding, a terminal V1 of the first V-phase winding and a terminal W1 of the first W-phase winding are respectively connected to U, V and W terminals on a power source side 31. Thus, the number of turns in each phase comes to the sum of turns of two windings (10, 11; 12, 13; 14, 15). This connection is suitable for attaining the characteristics for low-speed operation.
Referring to FIG. 9B, a terminal U2 of the second U-phase winding, a terminal V2 of the second V-phase winding and a terminal W2 of the second W-phase winding in the windings in Y-connection shown in FIG. 9A are respectively connected to the U, V and W terminals on the power source side 31. Thus, the number of turns in each phase becomes equal to the number of turns of a single winding (11, 13, 15), so that the number of turns in each phase is less than in the case shown in FIG. 9A, and as a result, this connection is suited for obtaining the characteristics for high-speed operation.
Next, referring to FIGS. 10A and 10B, a description will now be given of a prior art, in which a reactor is interposed between a power source and a motor in switching between Y-connection and .DELTA.-connection shown in FIGS. 8A and 8B.
Referring to FIG. 10A, a reactor 32 is interposed between U, V and W terminals on a power source side 31 and U, V and W terminals of a motor 33. A first switch 35 is interposed between the reactor 32 and the U, V and W terminals of the motor 33. Further, a second switch 36 is connected to Z, X and Y terminals of the motor 33.
In FIG. 10A, since the first switch 35 is at an off or open or closed position and the second switch 36 is at on position, the U, V and W terminals on the power source side 31 are respectively connected to the U, V and W terminals of the motor 33 through the reactor 32, and the X, Y and Z terminals of the motor are interconnected, so that Y-connection is formed. That is, a wiring shown in FIG. 10A is similar to that shown in FIG. 8A, except that the reactor 32 is interposed between the U, V and W terminals on the power source side 31 and the U, V and W terminals of the motor 33.
Referring to FIG. 10B, the first switch 35 in FIG. 10A is switched over to a closed or on-position, and the second switch 36 in FIG. 10A is switched over to an open or off-position. As a result, the U, V and W terminals on the power source side 31 are respectively connected to U and Z, V and X and W and Y terminals of the motor 33 through the reactor 32, so that .DELTA.-connection is set up. That is, a wiring shown in FIG. 10B is similar to that shown in FIG. 8B, except that the reactor 32 is interposed between the U, V and W terminals on the power source side 31 and the U, V and W terminals of the motor 33.
In the foregoing, as shown in FIGS. 10A and 10B, the reactor 32 is interposed between the power source side 31 and the motor 33 in both the cases where switching to Y-connection (FIG. 10A) is made for attaining the characteristics for low-speed operation and where switching to the .DELTA.-connection (FIG. 10B) is made for attaining the characteristics for high-speed operation. Thus, when the characteristics for high-speed operation are attained by the .DELTA.-connection, this reactor 32 is effective in suppressing the generation of heat by decreasing the higher harmonic component. On the other hand, when an attempt to attain the characteristics for low-speed operation is made through switching to Y-connection, the reactor functions so as to reduce output.